Human Population Growth
Extinctions and Humans
Factors adversely affecting species are
largely human-mediated and are occurring at
an extremely rapid and accelerating rate.
Smith et al. 2009. Ecology 90: 3279–3289.
Population modelers use different methods to
assess population growth.
1) exponential growth
dN
 rN
dt
N t  N 0 e rt
With exponential growth, the population rate of
change (r) is constant through time, while growth
of the population is geometric.
There is no adverse feedback on population
growth rate caused by increasing population size
and its concomitant reduction in remaining
resources
Exponential population growth cannot continue
indefinitely because organisms experience some
form of negative feedback.
2) logistic growth.
Logistic growth adds this feedback term to the
dN
equation of growth rate:
 K  N
 rN 

 K 
dt
where a is an integration
constant to define position and
K
on curve relative to origin
N 
t
1  e a  rt
Human population growth is affected by natality
and mortality rates. Throughout our history,
mortality rates have kept population growth at a
relatively low exponential growth rate of about
0.002% per year. Disease and famine were
particularly important.
Logistic Growth
Most of the increase has been due to compounding of
growth and to lower death rates.
Growth rates differ dramatically depending on whether
the country is affluent:
More Developed Countries [MDC]
or poor
Less Developed Countries [LDC]
AIDS and other diseases may impact growth rate
statistics in many countries, though particularly in LDC's
because many of the infected people are females of
child-bearing age.
Why do we care so much about population growth?
Simply put, each individual has an environmental
‘footprint’.
More mouths necessarily mean a greater demand of
environmental resources, but so does greater individual
use of resources.
For example, Postel et al. (1996) estimated that the
global human population now utilizes 54% of water
runoff that is geographically and temporally available.
Of course, access to potable water varies tremendously
on a global basis.
Water Shortages and the
southeastern United States
Atlanta
Vitousek et al. (1997) showed that application of
nitrogenous fertilizers has increased dramatically since
the 1940s; with other forms of human-mediated Nrelease, there has been a doubling of the nitrogen
entering the land-based N-cycle. This increase has a
adverse consequences including acid rain, loss of soil
nutrients (Ca, K), smog, and eutrophication.
Thus, human population growth has very profound
consequences for the characteristics of our
environment. Our ecological ‘footprint’ has been
calculated…
Biodiversity Hotspots
What are the leading hotspots?
Tropical Andes
Sundaland (Indonesia)
Madagascar,
Brazil's Atlantic forest
Caribbean islands
Each contains at least 2% of total plant biodiversity, or
a total of 20% of all plants and 16% of all
mammals. These regions are also among the world's
most impacted by human activities.
There appeared to be pretty good correspondence
between areas that were rich in plants and those rich in
vertebrates.
For example, areas rich in both plants and vertebrates
included the Philippines and various northern African
habitats, and the tropical Andes.
Low correspondence was found for The Cape region of
South Africa (rich only in plants - fynbos), and SW
Australia (rich in Acacia and Eucalyptus ).
Obviously the type of stress applied by humans will
differ from place to place. In the USA (and, almost
certainly, Canada as well) the major stresses imperiling
species are (Wilcove et al. 1998), in order:
1) habitat destruction and modification
2) nonindigenous species
3) pollution
4) overexploitation
5) diseases
Didham et al. 2005. TREE 20: 470–475.
Sala et al. (2000) examined global ecosystems and the
stresses expected to impact them over the next 100
years.
In streams, tropical forests and southern
temperate forests land use will be the major
factor affecting change.
In arctic and alpine ecosystems and boreal
forests, climate change will be the leading
factor.
In northern temperate forests, nitrogen
deposition will be most important.
1 = land use
2 = climate
3 = nitrogen deposition
4 = biotic exchange
5 = atmospheric CO2
Lakes and Mediterranean regions will be most
impacted by species invasions.
Independent of biome, by the year 2010
land use will be the most influential driver
affecting changes in biodiversity.
References
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Cincotta et al. 2000. Human population in the biodiversity hotspots. Nature 404:990992.
Didham et al. 2005. Are invasive species the drivers of ecological change? TREE 20:
470–475.
Hardin, G. 1968. The tragedy of the commons. Science 162:1243-1248.
Groom et al. 2005. Principles of Conservation Biology. Sinauer, Sunderland, MA.
Myers et al. 2000. Biodiversity hotspots for conservation priorities. Nature 403:853-858.
Postel et al. 1996. Human appropriation of renewable fresh water. Science 271:785788.
Reid, W.V. 1998. Biodiversity hotspots. TREE 17: 275–280.
Sala et al. 2000. Global biodiversity scenarios for the year 2100. Science 287:17701774.
Smith et al. 2009. A framework for assessing ecosystem dynamics in response to
chronic resource alterations induced by global change. Ecology 90:3279–3289.
Sisk et al. 1994. Identifying extinction risks. Bioscience 44:592-604.
Vitousek et al. Human alteration of the global nitrogen cycle: causes and consequences.
Issues in Ecology 1: 1-15.
Wilcove et al. 1998. Quantifying threats to imperiled species in the United States.
Bioscience 48:607-615.